This invention relates to sheathed electrical heating elements and more particularly to an electrical insulating refractory composition for use therein.
Sheathed electrical heating elements are used extensively in many heating applications. These elements consist of a metal sheath, an electrical heating element located within the sheath and an electrical insulating material embedding the heating element within the sheath. Generally, the embedding material is fused magnesium oxide which has excellent thermal conductivity while maintaining high electrical resistivity.
The high temperatures which are reached in such heating elements, their continued use over a long period of time, and the thermal cycling tends to degrade the insulating materials. For this reason, many different combinations of materials have been investigated with varying degrees of success. The object is to form an embedding composition which will be stable over a wide range of temperatures both as to electrical resistivity and thermal conductivity. Although there are materials which can be added to the magnesium oxide which will enhance these properties, there are other factors to be taken into consideration. More specifically, the embedding material must be able to be vibrated or tapped to a relatively dense material prior to compaction. This property is referred to as the "tap density" and it is measured by the ASTM Procedure No. 3347-74. The other property which is effected by additives is the flowability of the embedding material powder. It is necessary that adequate flowability be maintained so that the powder will flow through the machines which are normally used by the heating element industry. The current technique employed for manufacture of electrically insulating magnesium oxide powders includes grinding and sizing which reduces the magnesium oxide particle size dimension such that all particles will pass a U.S. Standard 40 mesh sieve (0.0165 inches). The particles are then polished by standard process which will increase the tap density of the powder. This is followed by calcining which increases the electrical resistivity.
Calcining is accomplished by heating the magnesium oxide powder to a temperature in excess of 1200.degree. C. Electrical resistivity is increased by the calcining process wherein oxygen deficiencies of the magnesium oxide crystal lattice are satisfied and oxidation of various impurity phases is completed. Because of the tendency to sinter at temperatures above 1100.degree. C., magnesium oxide powder loses a portion of its ability to flow and suffers a reduction in tap density because of the calcining process. These latter properties may be reduced to unacceptable levels during the calcining. Therefore, it is necessary to make a compromise with respect to the calcining process such that increased electrical resistivity can be obtained without overly reducing the tap density and flowability. This means that the maintenance of adequate tap density and flowability requires that electrical resistivity be accepted which is lower than the potential maximum.